1. Field of the Invention
This invention relates to apparatus and an associated method for inspecting containers for leakage and if desired to determine if the volume is as desired.
2. Description of the Prior Art
The inspection of containers in order to confirm that they have the desired volume and do not leak can be extremely important to both manufacturers of containers and those who fill the containers with products to be sold.
Leaking containers not only can result in undesired loss of or contamination of the product contained therein, but also can result in the product which is leaking out of the container damaging or destroying other property or injuring people.
It is also very important to manufacturers that a container have the proper volume in order that it will hold the labeled amount of product. Even if there are volume variations, the product manufacturer generally desires to have all of the containers filled to nearly the same height. Departures from a predetermined range of desired volume capacities can interfere with this objective.
It has been known to measure volume of a container using gravimetric means wherein an empty container is weighed and the container is then filled with water and weighed again. Assuming that the water is of known temperature and is free of dissolved air, one may easily calculate the volume of the container. This approach, however, is too time consuming for use in rapid inspection of containers and is susceptible to human error.
It has also been known to compare a container of unknown volume or test container to another of the same nominal size and shape which has had its volume measured carefully by gravimetric means. One of the problems with such an approach is that measurements can be in error if the test containers are a few degrees above or below the ambient temperature. Further, such systems are less accurate when the containers are wet as through condensation. In addition, the test can take several tenths of a second to perform which means that only about 100 containers can be tested per minute.
It has also been known to seek to detect leaks in containers by employing pressure-decay techniques. In such an approach, a known pressure is applied to a container and the container is sealed. If a leak is present, the pressure in the sealed container will decrease over time. A pressure transducer may be used to measure the size of the leak. Such approaches require either long decay times, or if small leaks are to be detected, the use of high pressure. U.S. Pat. No. 3,527,909 discloses a differential pressure switch which is said to be useful in leak testing. This patent illustrates a conventional pressure-decay system.
It has been known to employ Boyle's law in inspecting containers under isothermal conditions. See generally, U.S. Pat. Nos. 3,060,735 and 3,113,448.
U.S. Pat. No. 3,075,382 discloses the use of a single pressure source and two volumes and a continuous pressure fluctuation which in this instance is an acoustical device in measuring volume. This system also will be limited by temperature variations as the velocity of sound is dependent on air temperature. This patent also contains an allusion to use of the apparatus as a means for determining the presence of a leak at the seal between the apparatus and the container. U.S. Pat. No. 3,241,361 also discloses an isothermal method of measuring volume . This system discloses the use of a static pressure source and a manual pressure balance. See also U.S. Pat. No. 4,083,228.
U.S. Pat. No. 3,921,436 discloses leak testing apparatus for inspecting automotive heat exchangers. This system employs reference and test objects of similar shapes and says that they are to be maintained at the same temperature. See also U.S. Pat. No. 4,686,638 which discloses the use of a moving average technique to minimize the effects of temperature fluctuations. This patent also points out the difficulty of attempting to employ an isothermal system under non-isothermal conditions which frequently exist in plant environments.
U.S. Pat. No. 2,849,881 discloses the use of a continuously operating pump and servo-balance system for balancing pressure between two sides of the system in measuring liquid quantity in a closed tank. A pair of pneumatic resistors are positioned between the closed volume and the closed space. The system contemplates isothermal operation.
In spite of the foregoing prior art disclosures, there remains a very real and substantial need for a rapid means for determining whether a container leaks and if its volume corresponds to a desired predetermined volume, particularly in view of the practical difficulty of maintaining isothermal conditions.